I was informed by T. Johnston, Esq. of Exeter, that he had frequently examined the red ground in the vicinity of the different trap rocks in Devonshire, and that he invariably found it composed of fragments of these rocks, increasing in size as he approached nearer to them. The sand rock on which Nottingham and Nottingham Castle are built, has, evidently, been formed of the ruins of more ancient rocks in its vicinity; and the rounded pebbles of quartz and of Lydian stone, granite, porphyry, jasper, and mica-slate, indicate that they have come from rocks, formerly connected with the Charnwood Forest range. Still nearer the Charnwood hills, the finest sandstone contains fragments of slate, and the lower conglomerate is almost entirely composed of the fragments of the Charnwood rocks, as before observed. In the Vosges, the red sandstone every where accompanies the granitic and transition rocks, of which also it contains fragments. It must be recollected that the rocks most disposed to decompose or disintegrate, would be the soonest worn down. With the exception of the Malvern range we have no rocks of soft granite, or sienite in England, like those of Auvergne, or of the Forez mountains in France; and the reason why we have not, may be, that, from their smaller magnitude, they were probably carried away by those mighty inundations, that have swept over our present islands and continents. The Malvern Hills, the Lickey, the Charnwood Forest Hills, and the trap rocks in Gloucestershire, Somersetshire, and Devonshire, are the remaining nuclei of much larger ranges, as the scattered fragments in the adjacent, as well as in distant districts attest. If the red marl and sandstone in England, and in other countries, were formed of decomposing rocks of trap, granular quartz, porphyry, sienite, and granite, the frequent occurrence of porphyroidal beds in this formation may admit of a probable explanation. It is not intended to maintain, that every bed or stratum in this extensive formation is composed principally of the fragments of transition and trap rocks; but it may be safely affirmed, that there are few strata, in which some of these fragments may not be discovered. The red marl produces some of the most fertile soils in England, which may be owing partly to its formation from soft trap rocks. Some basaltic rocks decompose rapidly, and are known to form soil favourable to vegetation; several basaltic rocks in Staffordshire decompose into a reddish brown clay, moderately tenacious. A very remarkable discovery has been recently made (1828), of the foot-marks of some unknown quadruped in strata of new red sandstone, at the Corn Cockle Muir, three miles from Lochmaben in Dumfries-shire. They were found forty-five feet under the present surface; the strata are inclined thirty-seven degrees. This circumstance was communicated to the author by Mr. Murray, jun. of Albemarle Street, who showed him at the same time a plaster cast, taken from a slab of stone, in which the impressions were tolerably distinct, and also part of a thin stratum of the stone itself, with indis FOOT-STEPS IN RED SANDSTONE.-SUCCESSION OF STRATA. 167 tinct impressions of a similar kind. There can be scarcely a doubt, that they were the real foot-marks of a digitated animal having short toes and claws, and the foot broad in proportion to its length. The breadth of the foot is above one inch. The part of the sandstone in Mr. Murray's possession, appeared to be composed principally of granular fragments of reddish quartz rock and felspar, with spots of chlorite or hornblende. As remains of reptiles have been found in the zetchstein or magnesian limestone on the Continent, which is as ancient as the new red sandstone, may not this animal have been a reptile allied to the tortoise? Since the publication of the third edition of this work, Professor Buckland has, I believe, ascertained that the foot-marks are similar to the foot-marks which some species of tortoise make in sand. Before concluding the account of the red sandstone, it may be proper to repeat, that in a formation of such complexity, it is often difficult to determine to which part of the series any particular bed belongs, unless its situation be indicated by some of the limestone beds, which sometimes occur in different parts of it. Thus, in Devonshire, the porphyritic beds and conglomerates may belong to the lowest, or to the middle series of sandstones: their position, with respect to the rocks on which they rest unconformably, does not assist in the discovery. In Yorkshire, the very lowest series rest on coal measures, as stated by Professor Sedgwick, in his masterly and luminous description of the geological relations of the magnesian limestone, from Northumberland to Nottinghamshire. At Charnwood Forest, the uppermost series rests on ancient granitic and slate rocks, as represented in Plate III. fig. 4. a, a. In the lowest beds, resting on the slate, I observed indications of their mode of formation, which I intend afterwards to describe. Professor Sedgwick first ascertained the true relations of the lower sandstone; but, twenty years before, in the first edition of this work, (1813, p. 270.) I gave a brief account of the Pontefract sand rock, as the last of the rock formations over coal, in a description of a section from the Yorkshire to the Lancashire coast:-"The magnesian limestone is succeeded by yellow siliceous sandstone, on which the town of Pontefract is built. We may consider this as the boundary of the low calcareous district: proceeding in a direction to Wakefield, we soon come upon the argillaceous coal strata of the middle district." Professor Sedgwick arranges the red sandstone and magnesian limestone in an ascending series. 1. Lower red sandstone, yellow and red. 2. Marl slate and compact limestone. 2 a. Compact and shelly limestone, and variegated marls. 3. Yellow magnesian limestone. 4. Lower red marl and gypsum. 5. Upper thin bedded limestone. 6. Upper red sandstone. 7. Upper red marl and gypsum. It may deserve notice, that the red sandstone generally occupies the depressions in the more ancient strata, or what were once deep valleys, and also fills up hollows on the surface of ancient rocks, as represented in Plate III. fig 4. a a. Now, as these depressions and hollows were, originally, filled up when the surface was under the ocean, and are now raised some hundred feet above its present level, without any apparent disturbance, this fact proves, that there were two elevating causes acting at different epochs,-the first violent and transitory, which tilted up the lower beds; the second, more extensive, but more gradual in its operation, which upheaved the whole country above the ocean, and formed islands and continents. Magnesian Limestone.-The geological position of this rock is over the lowest beds of new red sandstone; but where this is wanting, it lies unconformably over the regular coal formation: see Chap. VIII. It is covered by the middle and upper series of new red sandstone. The dolomite found in primitive and transition rocks has been before described; it is commonly white, or light grey and granular. That in the secondary strata has generally a dark brown or a yellowish-brown colour: it contains a variable proportion of magnesia, sometimes more than fifty per cent. The presence of magnesian earth, in the proportion of nearly one half, in certain limestones, is a fact that strongly militates against the theory, which ascribes the formation of all limestone rocks to animal secretion; unless it shall be found that magnesian earth is contained in the shells and exuviæ of marine animals. I believe no analyses of shells or coral have yet been made, in order to ascertain the presence of magnesia as one of their constituent elements. Should magnesia be found in the exuvia of certain orders of marine animals, and not in others, it would not only favour the opinion that limestone was of animal origin, but might also explain the cause of the alternation of beds of magnesian limestone with beds of common limestone, in the same mountain. Or should some shells of one species contain magnesia, and others none, it would prove that, under different circumstances, the same animal might form its shell of different constituent parts. Professor Sedgwick is inclined to derive the magnesian limestone from the debris of beds of mountain or transition limestone which contain magnesia; but many beds of the magnesian limestone, above the coal formation, have as much the character of original rocks as the beds of transition limestone, and the difficulty is not removed by this hypothesis; for it still remains to enquire, from whence did the mountain or transition limestones derive their magnesia? Von Buch ascribes the change of the common limestone into dolomite in the Tyrol, to the action of volcanic rocks and volcanic vapours containing magnesia; but this opinion is not likely to obtain many supporters. Can the magnesia found in some of the chalk rocks in England or France be derived from volcanic rocks? Were the theory of Von Buch true, we ought to expect all limestone rocks in the immediate vicinity of basalt to be magnesian; but some experiments which I made on the mountain limestone of Derbyshire, in near proximity to the toadstone, proved that it did not contain so much magnesia, as the beds that were much farther removed from the latter rock. The magnesian limestone is distinctly stratified; the strata vary in thickness from a few inches to several feet: in the northern counties of England they are nearly horizontal; they border the great coal formation, and cover it on the eastern side. This formation of limestone extends from the mouth of the Tyne to near Nottingham. The colour of the limestone is generally a yellowish or reddish brown, varying in intensity from a fawn color to that of an overburnt brick. Some of the lowest beds are bluish and slaty, and intermixed with marl; but these beds seldom rise to the surface, and their nature is little known. Some beds of magnesian limestone have a granular sandy structure, others are imperfectly crystalline: they possess a considerable degree of hardness. A cellular variety of this limestone occurs near Sunderland, which has received the name of Honeycomb limestone: it agrees in most of its characters with the rauch wacke of Thuringia, which is part of the zechstein formation. Many beds of magnesian limestone yield a fœtid smell when rubbed. At Sunderland, the beds of magnesian limestone are more developed than in any other part of England that I am acquainted with. In an account which I published of the Geology of Durham, in the Philosophical Magazine for 1815, I estimated the total thickness at one hundred and fifty yards. This limestone has been bored into to a considerable depth below the surface; it was, as before mentioned, of a bluish colour. According to Mr. Farey, "under the yellow beds of magnesian limestone, there are several beds of compact blue limestone, abounding with Anomia (Terebratulæ) and other shells; some of these beds differ entirely from the yellow and red beds, and are more useful for agricultural purposes, particularly on the yellow limestone lands."* This is the marl slate of Professor Sedgwick. The lower beds of this formation are, I believe, more fully developed in many parts of the Continent than in this country, which occasions some uncertainty in classing them. The limestone of Thuringia, it is agreed by the most respectable geologists, is zechstein, corresponding with our magnesian limestone; the lower part is a slaty marl, sometimes impregnated with bitumen, and sometimes with sand. This bed contains impressions of fish, like the lower beds of the slaty Sunderland magnesian limestone: it contains also a small quantity of copper pyrites, and the ores of lead, cobalt, zinc, bismuth, and arsenic, and is in some places worked by the miners for its mineral treasures. Above this bed there occurs a blackish-grey * Survey of Derbyshire, p. 157. compact limestone, very hard and tenacious, and distinctly stratified; over this is a cellular limestone; and above this, a blackish brown limestone, which yields a fœtid smell when struck with a hammer, and is in some places more than one hundred feet in thickness. All these different beds Humboldt comprises under the name of zechstein, and agrees with other geologists in referring them to our magnesian limestone; the lowest bed rests on the red sandstone, and sometimes alternates with it but according to some geologists, the connection between the two formations of red sandstone and zechstein is such, that the latter may be regarded as a subordinate formation to the former. The upper beds of what has been called zechstein alternate in Switzerland with beds of gypsum, which is intermixed with rock-salt: some of the beds are argillaceous limestone, containing ammonites and belemnites, and appeared to me to have a greater resemblance to lias, than to magnesian limestone. In the lower part of the magnesian limestone in the West of England, there is a conglomerate limestone, which contains fragments of transition limestone, varying in size from several inches in diameter, to very minute grains. The fossils in magnesian limestone are not numerous, at least in the upper beds. Fossil fish have been found in some of the lower beds in the county of Durham. One or two species of univalves, and about nine species of bivalves, occur in this limestone; but these shells are extremely rare, except in one or two situations. Some of the shells, the productus and spirifer, nearly resemble those in the mountain limestone, to which the magnesian limestone appears to bear a greater analogy, than to any of the secondary strata above it. Magnesian limestone furnishes the most durable building stone that is any where found in the upper secondary strata. I do not agree in opinion with those who regard the magnesian limestone districts as unfertile; and perhaps no parts of England are more salubrious, than those which have a subsoil of this limestone. A few small strings of lead ore have been found in the magnesian limestone rocks near Sunderland. The limestone rocks on the coast of Durham are wearing away by the violence of the ocean: they have evidently extended much further to the east than at present. It has already been stated, that beside magnesian limestone, gypsum and rock salt are associated with the new red marl and sandstone. Neither of these minerals is however confined to this formation. Salt springs rise in many of the coal strata, and gypsum and rock-salt are found both in the upper, secondary, and the tertiary beds; but the repositories of these minerals are more characteristic of the new red sandstone, and may therefore, with propriety, be described in the present chapter. Gypsum, both fibrous and massive, occurs in the new red marl and sandstone: the fibrous gypsum forms numerous alternating seams in cliffs of red marl: the seams vary in thickness, from one to |